1. Field of the Invention
The present invention relates generally to collectors for secondary batteries and the secondary batteries, and particularly to metal foil as a material of collectors for lithium ion batteries and polymer lithium ion batteries and methods of manufacturing the same.
2. Description of the Background Art
In recent years, lithium ion batteries and polymer lithium ion batteries have been used as a secondary battery of high energy efficiency to supply power to various types of electronics.
A conventionally used collector for a secondary battery is formed of a metal foil having round or square throughholes, with a pasted active material for example of an interlayer graphite compound, a conductive polymer compound, activated carbon, a carbide of phenol-based fiber, and a lithium-based compound applied and processed and thus adhering the pasted active material to a surface of the metal foil. The metal foil has a throughhole diameter or size of approximately 0.5 to 1 mm and a hole area ratio of approximately 20 to 40%. A punching metal pattern is used in providing the throughholes in the metal foil.
Conventional metal foil for a collector is provided with multiple throughholes to increase the foil""s surface area to improve an active material""s adhesion thereto. Forming such throughholes having as large a diameter as possible at as high a density as possible can increase the foil""s surface area and thus enhance the active material""s adhesion to the foil.
However, if conventional round or square throughholes provided in a metal foil have a diameter of 1.55 mm or more and a hole area ratio of 40% or more, an active material can peel off or drop from the foil at a center of such round or square throughholes, resulting in a disadvantageously degraded adhesion of the active material to the foil. As such, any throughhole with a conventional shape is limited in diameter or size and hole area ratio.
Furthermore, providing a metal foil with conventional, round or square througholes and thus achieving an increased hole area ratio disadvantageously reduces the foil""s strength. In particular, a metal foil for a collector can disadvantageously be torn apart or deform when it is prepared or an active material is applied thereto.
In conventionally preparing a metal foil for a collector, a punching metal pattern is used to provide throughholes therein. When such throughholes are punched in the metal foil for a collector, formed of a so-called punching metal, the metal foil""s dust is disadvantageously introduced into the product or the foil is burred, resulting in an electrical short circuit internal to a secondary battery as a final product. This degrades the quality of the secondary battery as the final product. Furthermore, providing a metal foil with multiple fine throughholes requires a punching metal pattern with high precision dimensions. This requires a cumbersome maintenance of the metal pattern and other tools and equipment and thus disadvantageously increases the cost for preparing the metal foil and decreases the productivity thereof.
An object of the present invention is to provide a metal foil for a collector having a plurality of throughholes having a largest diameter of at least 1.55 mm and a shape allowing a hole area ratio increased to at least 40% to enhance an active material""s adhesion thereto.
Another object of the present invention is to provide a metal foil for a collector having multiple throughholes having a largest diameter of at least 1.55 mm and achieving a hole area ratio of at least 40% to enhance an active material""s adhesion thereto, wherein the throughholes are arranged to allow the foil to have a predetermined strength in a certain direction.
Still another object of the present invention is to provide a method of preparing a metal foil for a collector, capable of readily providing a throughhole allowing enhanced adhesion and improved quality and increased hole area ratio.
Further, another object of the present invention is to provide a collector with a metal foil capable of enhancing an active material""s adhesion thereto.
Yet another object of the present invention is to provide a secondary battery with a collector capable of enhancing an active material""s adhesion thereto.
In accordance with the present invention a metal foil for a collector has a plurality of throughholes having a largest diameter or dimension of at least 1.55 mm, a hole area ratio of at least 40% and a thickness of at least 5 xcexcm and at most 200 xcexcm, wherein the throughhole has a peripheral shape including two substantially straight sides extending along lines intersecting each other at an acute angle to form a corner of the throughhole.
The throughhole""s peripheral shape as described above allows the throughhole to be formed with a large diameter or dimension to provide an increased hole area ratio and also achieves an enhanced adhesion of the active material to the foil. In particular, providing the throughhole""s acute corner as described above is more effective than providing a conventional round or square throughhole in preventing the possibility of the active material peeling off or dropping from the foil, to allow the active material to have a better adhesion to the foil.
Preferably the throughhole""s peripheral shape as described above has a plurality of corners.
Furthermore, in accordance with the present invention a metal foil for a collector has a plurality of throughholes having a largest diameter of at least 1.55 mm, a hole area ratio of at least 40% and a thickness of at least 5 xcexcm and at most 200 xcexcm, wherein a continuous, elongate portion having a width of at least 0.1 mm and at most 2 mm extends in a certain direction between the plurality of throughholes.
The continuous, elongate portion as described above ensures that a metal foil for a collector can have a strength of at least a predetermined value if a large number of throughholes having a large diameter are formed to provide an increased hole area ratio. In rolling a metal foil to prepare it in the form of a strip and thereafter providing it with throughholes, the foil is required to have a predetermined tensile strength in a direction in which it is rolled. In accordance with the present invention, a predetermined, continuous, elongate portion extending in a certain direction can prevent a metal foil from being torn apart or deforming when the foil is initially rolled and then provided with a large number of throughholes satisfying the hole diameter and hole area ratio as above.
Preferably, the metal foil has in the certain direction a tensile strength of at least 6N when it has a width of 15 mm (6N/15 mm width).
Preferably, the metal foil is etched to provide the throughholes.
In accordance with the present invention, preferably a metal foil for a collector contains at least one of aluminum and copper.
In accordance with the present invention, still further preferably a metal foil for a collector has a plurality of throughholes having a largest diameter or dimension of at least 1.55 mm, a hole area ratio of at least 40% and a thickness of at least 5 xcexcm and at most 200 xcexcm, wherein the throughhole has a peripheral shape including two substantially straight sides extending along lines intersecting each other at an acute angle to form a corner of the throughhole, and a continuous, elongate portion having a width of at least 0.1 mm and at most 2 mm extends in a certain or selected direction between the plural throughholes.
Furthermore, in accordance with the present invention a collector for a secondary battery includes a metal foil characterized as described above.
Furthermore, in accordance with the present invention a secondary battery includes the collector as described above and preferably includes a positive electrode including a metal foil containing aluminum and characterized as described above, and a negative electrode including a metal foil containing copper and characterized as described above.
In accordance with the present invention a method of preparing a metal foil for a collector includes the steps of providing on a metal foil a resist film patterned as predetermined, etching the foil, with the resist film used as a mask, to provide a plurality of throughholes in the foil, and removing the resist film.
With the method as described above, when throughholes are formed any dust or burr does not result and a second battery as a final product is not adversely affected in quality. Furthermore, the method eliminates the necessity of cumbersome maintenance of a tool and equipment for providing a large number of throughholes and thus does not increase the cost for manufacturing the same or reduce the productivity thereof.
In the above method, preferably an acid or alkaline solution is used to etch the foil.
As described above, the present invention can provide a metal foil for a collector with a large hole area ratio and a high tensile strength in a certain direction. As such, there can be provided a metal foil for a collector or a collector which allows an active material to be reliably applied with good adhesion onto a surface of the foil and hardly disadvantageously creeps or deforms if the secondary battery is used for a long period of time.
Furthermore, in the present method of preparing a metal foil for a collector, a throughhole is provided by etching the foil, rather than mechanically processing the foil. The metal foil is thus free of a burr or dust resulting from punching a throughhole. This can prevent electrical short circuit and other deficiencies in using the secondary battery as a final product.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.